This invention relates to an improved method of converting mixtures comprising major amounts of methane and minor amounts of normally gaseous higher hydrocarbons to normally liquid hydrocarbons. More particularly, the invention relates to an improved method of converting at least a substantial portion of methane-containing gaseous hydrocarbon mixtures to liquid hydrocarbons.
Natural gas is typically a mixture of about 70 mole percent to about 98 mole percent of methane mixed with heavier hydrocarbons. These heavier hydrocarbons are mostly alkanes of two or more carbon atoms present in decreasing amounts according to increasing carbon number. Although these heavier hydrocarbons are typically present in small quantities relative to the methane and other normal gaseous materials such as carbon dioxide and even hydrogen sulfide are often present, as used herein, the term xe2x80x9cnatural gasxe2x80x9d refers to a mixture which is predominantly methane with from about 2 mole percent to about 30 mole percent of other normally gaseous hydrocarbons, e.g., ethane, propane and butane. Removal of any other materials, such as carbon dioxide or water, if necessary, is accomplished by conventional methods.
Natural gas is an important energy source throughout much of the developed world including the United States. Particularly important is the use of nature gas as an energy source in its property of clean burning without the co-production of environmentally damaging oxides of nitrogen and sulfur. Unfortunately, the location of natural gas usage is often distant from the location of natural gas production so that transportation of the natural gas as by pipeline, or electrical energy produced therefrom, is often difficult and/or prohibitive because of the cost of such transportation.
It has long been desirable to convert natural gas to a liquid product which would facilitate transportation. Conventional methods for such conversion include liquefaction of the natural gas, partial oxidation of the natural gas to produce methanol and utilization of Fisher-Tropsch technology to convert methane to mixtures of carbon monoxide and hydrogen which are then converted to light olefins and paraffinic hydrocarbons. The process of U.S. Pat. No. 3,156,733 includes the pyrolysis of methane to produce acetylene and hydrogen followed by rapid quenching.
In copending U.S. patent application Ser. No. 09/199,502, filed Nov. 25, 1998, of Hall, et al., a thermal process is disclosed in which the natural gas is heated to a temperature where a portion of the methane component of the natural gas is cracked or pyrolyzed to produce hydrogen and reactive hydrocarbons such as ethylene and acetylene. This stream, also containing methane, is reacted in the presence of an acidic catalyst to produce a transportable liquid product which is predominantly pentane. Illustrative of the pentane production are the simplified equations which follow.
3 CH4+C2 H2xe2x86x92C5H12+H2
3 CH4+C2H4xe2x86x92C5H12+2 H2
Unfortunately, the elevated temperatures required to crack the methane component is sufficiently high to destroy a portion of the higher hydrocarbons present in the natural gas as well as a portion of the reactive hydrocarbons produced by methane cracking through the formation of coke. This loss of potential liquid products of the overall process as by coke formation results in loss of economy of the process.
It would therefore be of advantage to provide an improved thermal process for the conversion of natural gas, but also of other methane-containing gaseous hydrocarbon mixtures, to normally liquid, more easily transportable, hydrocarbons in which the loss of hydrocarbons of two or more carbon atoms is minimized and the efficiency of the production of transportable liquid hydrocarbon is increased.
The present invention provides an improved method for the conversion of methane-containing gaseous hydrocarbon mixtures into normally liquid hydrocarbons of increased ease of transportation. The process includes the separation of the methane component of the methane-containing gaseous hydrocarbon mixture from the heavier hydrocarbon component. In the process of the invention, this heavier hydrocarbon component is subjected to cracking at a relatively low cracking temperature sufficient to crack the heavier hydrocarbons, but not crack an appreciable quantity of methane. This low temperature cracking process minimizes the loss of heavier hydrocarbons as by coking with a resulting overall efficiency of liquid product formation.
The methane-containing gaseous hydrocarbon mixtures to which the invention relates include natural gas but also the light gases that are produced in petroleum refinery operations such as catalytic cracking and delayed coking. Gaseous hydrocarbon mixtures resulting from these operations are typically separated to recover the heavier, more valuable components with the light ends being used for fuel. Such streams often contain a significant portion of ethane, propane, alkanes and alkynes and hydrogen in addition to methane. The process of the invention is useful to convert these xe2x80x9cwaste gasxe2x80x9d streams to more valuable liquid products.
The methane-containing gaseous hydrocarbon mixtures to which the process of the invention applies are those normally gaseous hydrocarbon mixtures containing at least about 40 mole percent methane and preferably at least about 55 mole percent methane with the remainder comprising saturated and unsaturated hydrocarbons as well as hydrogen. Of primary interest, as the methane-containing gaseous hydrocarbon mixture is natural gas. Natural gas, as obtained from the ground, often contains non-hydrocarbon materials such as water, hydrogen sulfide and carbon dioxide. As used herein, however, xe2x80x9cnatural gasxe2x80x9d is used to describe a mixture of about 70 mole percent to about 98 mole percent of methane with the remainder being heavier hydrocarbons, mostly alkanes, of two or more carbon atoms present in decreasing amounts according to increasing carbon number.
In certain embodiments of the process of the invention, at least a major proportion of the separated methane is separately cracked in the substantial absence of the heavier hydrocarbon component of the natural gas feed. Remaining portions of the separated methane are reacted with unsaturated hydrocarbons in the presence of an acid catalyst to produce the normally liquid hydrocarbon product as illustrated by the above equations.
In another embodiment of the process of the present invention, the separated methane is not cracked but is in part used for the acid-catalyzed reaction of methane with cracked heavier hydrocarbons with any methane not so employed being removed from the process system and sent to disposal or used in the production of thermal or electrical energy as by conventional methods. Such energy is useful in portions of the overall process of the invention or in other applications.
In a specific embodiment of the invention, a novel method of separating the methane portion of the natural gas feed from the heavier hydrocarbon component is employed.